37: Voluntary Movement: The Primary Motor Cortex

Introduction

• Motor Functions Are Localized within the Cerebral Cortex

• Many Cortical Areas Contribute to the Control of Voluntary Movements

  • Voluntary Motor Control Appears to Require Serial Processing

  • The Functional Anatomy of Precentral Motor Areas is Complex

  • The Anatomical Connections of the Precentral Motor Areas Do Not Validate a Strictly Serial Organization

• The Primary Motor Cortex Plays an Important Role in the Generation of Motor Commands

  • Motor Commands Are Population Codes

  • The Motor Cortex Encodes Both the Kinematics and Kinetics of Movement

  • Hand and Finger Movements Are Directly Controlled by the Motor Cortex

  • Sensory Inputs from Somatic Mechanoreceptors Have Feedback, Feed-Forward, and Adaptive Learning Roles

  • The Motor Map Is Dynamic and Adaptable

  • The Motor Cortex Contributes to Motor Skill Learning

• An Overall View

One of the main functions of the brain is to direct the body's purposeful interaction with the environment. Understanding how the brain fulfils this role is one of the great challenges in neural science. Because large areas of the cerebral cortex are implicated in voluntary motor control, the study of the cortical control of voluntary movement provides important insights into the functional organization of the cerebral cortex as a whole.

Evolution has endowed mammals with adaptive neural circuitry that allows them to interact in sophisticated ways with the complex environments in which they live. Adaptive patterning of voluntary movements gives mammals a distinct advantage in locating food, finding mates, and avoiding predators, all of which enhance the survival potential of the individual and a species.

The ability to use fingers, hands, and arms in voluntary actions independent of locomotion further helps primates, and especially humans, exploit their environment. Most animals must search their environment for food when hungry. In contrast, humans can also "forage" by using their hands to cook a meal or simply punch a few buttons on a telephone and order takeout. The central neural circuits responsible for such nonlocomotor behavior emerged from and remain intimately associated with the phylogenetically older circuits that control the forelimb during locomotor behaviors.

In this and the following chapter we focus on the control of voluntary movements of the hand and arm in primates. In this chapter we describe the cortical networks that control voluntary movement, particularly the role of the primary motor cortex in the generation of motor commands. In the next chapter we address broader questions about cortical control of voluntary motor behavior, in particular how the cerebral cortex organizes the stream of incoming sensory information to guide voluntary movement.

Voluntary movements differ from reflexes and basic locomotor rhythms in several important ways. By definition they are intentional—they are initiated by an internal decision to act—whereas reflexes are automatically triggered by external stimuli. Even when a voluntary action is ...